Genetic mutations, environmental factors, such as maternal infections during pregnancy, as well as postnatal complications, are all potential causes of hearing loss in infants. While genetic mutations account for between 50–60% of these cases, environmental factors account for more than 25% of hearing loss among infants.
In some cases, a combination of genetic elements and environmental factors could lead to hearing loss. Some medicines are also known to cause hearing loss; however, these agents mostly affect people with certain genetic mutations. When a single person in a family suffers from hearing loss, it is denoted as sporadic; comparatively, when this condition runs in the family, it is referred to as familial hearing loss.
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Genetic mutations
There are more than 400 known genetic mutations that could result in hearing loss. The most common forms of genetic hearing loss are due to the dominant and recessive genes, of which 15% of cases are due to dominant genes, 70% from recessive, and the remaining 15% result from other modes of inheritance.
The gene encoding Connexin 26 (CX26) is typically the primary cause of recessive genetic hearing loss. Some individuals with hearing loss are also blind. Syndromic and non-syndromic are two kinds of hearing impairments that are prevalent among individuals with hearing loss, accounting for 30% and 70% of these patients, respectively.
Syndromic
Depending on the method by which a particular gene is passed from one generation to the other, syndromic hearing loss can be further classified as autosomal dominant, autosomal recessive, X-linked recessive, multifactorial, and mitochondrial inheritance syndromic hearing loss.
Autosomal dominant
A child has a 50% possibility of inheriting the gene with the dominant mutation if one parent has the condition. Waardenburg syndrome (WS) is the most common form of autosomal dominant syndromic hearing loss. WS is reflected in variable degrees of sensorineural hearing loss and pigmentary aberration of the skin and eyes.
Penetrance is high in the Branchiootorenal syndrome (BOR), leading to conductive, sensorineural, or mixed hearing loss. Three types of stickler syndromes have been identified that attribute to progressive sensorineural hearing loss. A rare, treatable kind of syndrome is Neurofibromatosis 2 (NF2), in which hearing loss typically begins after 30 years and puts individuals at risk for a variety of tumors.
Autosomal recessive
In autosomal recessive syndromic hearing loss, a child has a 25% possibility of inheriting the gene with the recessive mutation when both parents are carriers of the condition. Usher syndrome, which is the most common type of this form of hearing loss, results in sensorineural and retinitis pigmentosa (RP).
The second most common type is Pendred syndrome, which leads to severe-to-profound sensorineural and euthyroid goiter. Patients affected by Jervell and Lange-Nielsen syndrome have syncopal episodes that may lead to unexpected death.
Biotinidase deficiency results in sensorineural hearing loss and also causes neurologic and cutaneous features to develop. The extremely rare Refsum disease causes severe progressive sensorineural hearing loss and RP.
Genetics of hearing loss
X-linked recessive
X-linked recessive syndromic hearing loss is passed through families through female carriers who do not have the condition; however, each offspring from her has a 50% possibility of getting the condition.
In Alport syndrome, until 10 years of age, hearing loss is not evident; however, the condition eventually progresses to sensorineural hearing loss of varying severity and glomerulonephritis, leading to end-stage renal disease. In the Deafness-dystonia-optic neuronopathy (DDON) syndrome, also known as Mohr-Tranebjaerg syndrome, the TIMM8A gene mutation is the cause, with this condition being characterized by sensorineural hearing loss.
Multifactorial
Multifactorial syndromic hearing loss is due to the combination of both genetic and nongenetic factors. The second child will have a 3–5% chance of the condition when the first child has a multifactorial condition.
Infants diagnosed with multifactorial conditions are usually born in families with no other affected family member. Hearing loss associated with age factors such as Presbycusis belongs to the multifactorial category.
Mitochondrial inheritance
In individuals with a mitochondrial disorder, the auditory system is affected in the auditory cortex and cochlea, which is a structure present in the inner ear that is the component most sensitive to mitochondrial dysfunction. This condition is passed to the child from the mother if she has a mitochondrial condition.
Approximately 30% of Mitochondrial Encephalopathy, Lactic Acidosis, and Stroke-like Episodes (MELAS) may be characterized by sensorineural hearing loss. Similarly, in Maternally Inherited Diabetes and Deafness (MIDD), Kearns-Sayre Syndrome (KSS), Myoclonic Epilepsy, and Ragged Red Fibers (MERRF), hearing loss is sensorineural.
Non-syndromic
In autosomal dominant type hearing loss, heterogeneity is high and the hearing loss does not have a particular single gene that is accountable for the hearing loss in the majority of cases. In contrast, in autosomal recessive type hearing loss, the variant GJB2 causes deafness in 50% of the world’s population.
X-linked nonsyndromic hearing loss is characterized by a severe prelingual hearing loss that is progressive. The variants in the gene subsets, which include MT-RNR1,MT-TSI, and MT-CO1, are responsible for the maternally inherited Mitochondrial Nonsyndromic Hearing Impairment. Infants are kept away from using aminoglycosides when the mother has the m.1555A>G pathogenic variant of MT-RNR1, as she may pass it to the child through these antibiotics.
Genetic counseling
Physicians can counsel individuals and family members on the mode of inheritance, implications, and possible genetic risks involved with infant hearing loss. The availability of prenatal testing before pregnancy will educate parents and assist in their decision-making process.
References
Further Reading
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